The present invention relates to a semiconductor thin film that is fabricated by a laser crystallizing method, as well as a thin film transistor and semiconductor devices, such as a liquid crystal display apparatus, an active matrix type liquid crystal apparatus, a solar cell and the like, which use the semiconductor thin film, and a method of fabricating the semiconductor thin film, the thin film transistor and the semiconductor devices.
The laser crystallizing technology is most widely viewed as a means to fabricate a crystalline semiconductor at a low cost, which crystalline semiconductor is of the type which is applied to a high performance thin film transistor, a high value-added liquid crystal display apparatus and a solar cell. Because the crystallization of a semiconductor thin film locally heats only the vicinity of the semiconductor surface by laser irradiation, a low-cost glass substrate and a low-cost organic resin substrate can be used for the supporting substrate, and, accordingly, the laser crystallizing technology contributes to a reduced cost. Since the laser-irradiated semiconductor is first liquefied and then solidified so as to be crystallized, a high-quality crystalline semiconductor having less defects can be obtained. One way to improve the film quality of the crystalline semiconductor is to increase the crystal grain size. By increasing the crystal grain size, the volumetric ratio of the crystal grain boundary, including defects, to the whole semiconductor film is decreased, and, consequently, the mobility of electrons and holes is improved. Further, a decrease itself in number of the defects improves the quality of the crystalline semiconductor. In regard to a way to increase the crystal grain size, (1) Dig. Of Tech. Papers, 1997, Int. Workshop Active Matrix Liquid Crystal Display (Business Center of Academic Societies, Tokyo 1997), p59, proposes a method in which, after crystallizing an amorphous silicon by laser irradiation, an amorphous silicon film is formed on the fabricated polycrystalline silicon, and then the amorphous silicon is crystallized by laser irradiation.
On the other hand, a problem of the crystalline semiconductor, for example, the polycrystalline silicon fabricated by a laser crystallizing method, concerns the formation of an uneven surface due to many projections which are produced when a high-quality polycrystalline silicon having large crystal grain size is fabricated. The height of the projections is nearly equal to the film thickness of the semiconductor before irradiating the laser light. A mechanism for producing such projection is considered in the Applied Physics Letters, Vol.68, No.15, 1996, p2138, which indicates that the projections are formed by volumetric expansion caused by the phase transition from the melted silicon to the solid silicon at a boundary where surfaces of crystal growth in a direction lateral to the substrate surface collide with each other. The crystal growth in the lateral direction generally occurs when a crystal having a crystal grain size larger than the thickness of the semiconductor thin film is formed. When a semiconductor thin film having large unevenness is used to form the active layer of a coplanar type thin film transistor, the concentration of an electric field occurs at the projections so as to cause dielectric breakdown in the gate insulation film serving as the upper layer, of the active layer or to cause reduction of the reliability of the gate insulation film such as by production of defects due to hot carriers. In order to protect against these problems, the thickness of the gate insulation film needs to be formed so as to be thicker than 100 nm, and, consequently, it becomes difficult to drive the thin film transistor with low power consumption. Further, since the crystallinity of the projection is very low and the projection is located in a channel area when the semiconductor having many projections is used for a coplanar type or a normal stagger type thin film transistor, the ON current is reduced. In regard to techniques for suppressing the occurrence of the projections when the semiconductor thin film is crystallized with laser light, the following techniques have been reported.    (2) A method of irradiating laser light in stages with a pitch of 10 mJ/cm2 is described in IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol.42, No.2, 1995, p251.    (3) A method of irradiating laser light after poly-crystallizing amorphous silicon using a solid phase growth method is described in Dig. of Tech. Papers 1997, Int. Workshop Active Matrix Liquid Crystal Displays (Business Center of Academic Societies, Tokyo 1997), p167.    (4) A method of changing the shape of a laser beam so as to have a wide lower slopes is described in Shin-etsu Chemical Technical Report EID98-19 (1998-06), p67.
The above-described conventional technology (1) which proposes to increase the crystal grain size of the crystalline semiconductor has a problem in that the crystal grain size can be certainly increased, but projections having a height nearly equal to the film thickness of the semiconductor are produced, and, accordingly, a large unevenness is produced. Further, there is another problem in that, because the amorphous silicon before laser irradiation is exposed to the atmosphere in order to perform dehydrogenation and, thereby, a natural oxide film is formed on the surface, oxygen enters into the silicon film when it is crystallized by laser light to reduce the quality of the film.
On the other hand, the conventional technology (2) for suppressing production of the projections has a problem in that, since laser light is irradiated in stages with a small pitch of 10 mJ/cm2 and the fine crystalline silicon that is first formed is difficult to melt, what can be fabricated is only polycrystalline silicon having a crystal grain size of nearly 60 nm, and, accordingly, polycrystalline silicon having a large crystal grain size above 500 nm can not be fabricated. The conventional technology (3) for suppressing production of the projections has a problem in that, because the solid phase growth method is used and, consequently, the silicon is heated at 1000° C., an economical glass substrate can not be used, and, accordingly, the crystalline semiconductor can not be fabricated with a low cost. The conventional technology (4) for suppressing production of the projections has a problem in that, since the crystal grain size becomes small as the projection is made small, the small roughness and the large crystal grain size are not compatible with each other.